The present invention relates to improvements in foundations and methods of constructing the same. More particularly, the present invention relates to improvements in foundation structures and methods of constructing the same which reduce foundation failures and structural stresses caused by seasonal volume changes in supporting soil near the perimeter of the building as a result of fluctuations in the subsurface moisture content of the soil.
Expansive soils and the widely varying seasonal fluctuations in subsurface moisture content is a source of foundation distress in a very large number of residences. This is an especially severe problem in the Southern, Southwestern and Mid-western regions of the United States. Serious failures of foundation results in large expenditures on the part of homeowners. On the basis of previous studies, it has been estimated that an excess of $30 million per year is expended in foundation repair work in one metropolitan area alone. Nationwide, it has been estimated that at least 2.3 billion per year is expended to repair similar damage to houses, buildings, roads, and pipelines.
Although volume changes in all soils present problems with foundations for light structures, these problems are especially severe in areas where soils of the high-shrink-swell type (a plasticity index above 30) are found. Typical soils of this type are Houston Black Clay, soils yielded by the Taylor Marl and Eagle-Ford shale. These clays typically have a high montmorillinite content which causes the soils to have the extreme expansive characteristics.
In areas where high-shrink-swell soils are found, it is believed that many instances of building foundation stress is a result of migration of moisture from beneath the perimeter of the beams of the foundation, with resulting shrinkage of the soils and a loss of soil support at the perimeter of the foundation. This shrinkage occurs after extended periods of dry weather, common to the summer months in the South, Southwest and Midwest regions of the United States where evaporation by dry, hot winds and transpiration of plants deplete the moisture in the soil surrounding the foundation. The loss of moisture from the soil surrounding the foundation causes moisture in the protected area under the foundation to move by capillary action into the depleted surrounding soil whence it, too, may be transported into the atmosphere. This reduction of moisture content of the soil under the edges of the foundation causes the soil to shrink and move away from the foundation, thus, removing support under the exterior beams of the foundation. This removal of support under the exterior beams of the foundation can cause the foundation, masonry and interior sheetrock to be ruptured, doors and windows to be cracked and serious distresses to be induced in the structure of the building.
When the normal fall and winter rains replenish the soil moisture, the cycle is reversed. Moisture then migrates back under the foundation perimeter, swelling the supporting clays and moving the perimeter beams in a vertically upward direction. The perimeter of the foundation will continue to move up and down with the seasonal cycle with the magnitude of movement varying with the extremes of the wet-dry weather cycles.
These cyclic variations in soil volume are especially detrimental in conventional light structure building techniques wherein the structure is supported from a light slab type foundation. These light slabs tend to fail and deform under these conditions. Typical examples of these structures are homes, small buildings, canals, roads, railroads, pipelines, and the like.
The time of year when the building is constructed can also be an important factor, for if the foundation is formed during a wet period, the next seasonal dry period will cause moisture to move from under the edges of the foundation, causing the soil to shrink away from the foundation, removing support from the perimeter thereof. Conversely, if the foundation is constructed during the dry period, the next seasonally moist period will cause water to move by capillary action under the periphery of the foundation, swelling of the soil thereunder, and tending to lift the edges of the foundation.
Not only is the problem of shrinking and swelling of soil around the perimeter of the foundation present in the construction of new foundations, it is also present in the foundations of existing structures.
To solve these problems, a variety of corrective techniques have been used and the results vary widely. One technique is to utilize a massive slab and beam structure which will not fail as the soil shrinks and expands. Although, in some instances, these techniques may be successfully utilized, they are undesirable in that they are inordinately expensive and increase the construction costs in these structures. In addition, this technique cannot be used on existing foundations.
A second technique is to attempt to repair the damage to the existing foundations by grouting the cracks formed in the foundation due to the shrinking and swelling of the soil. These attempts tend to increase the damage to the foundation when the cycle is reversed in that the foundation cannot return to its original position because of grout material in the crack.
Other techniques have included attempts to stabilize the expansive soil present under a foundation. Stabilization techniques utilize lime slurry material which is injected into the soil under high pressure. The effectiveness of the use of this lime material is variable and believed to be a result of a chemical reaction that takes place between lime and clay whereby some types of clay particles are significantly altered and reduced in plasticity and increased in mechanical strength. It was believed that this chemical reaction could be achieved throughout the soil by mixing lime with the soil under the foundation. However, it has been found that such mixing is very limited. The process is quite expensive and the results are not predictable.
Another technique was the use of a subsurface irrigation system positioned under the foundation to control the moisture content of the soil under the foundation. In this technique, permeable hoses or pipe similar to the type used in lawn irrigation systems are placed in the soil of the foundation. A low head pressure of water was then connected to the pipes causing water to slowly percolate into the soil supporting the foundation. The irrigation system can be placed adjacent to the edge of the foundation and moisture sensors imbedded around the house to control the operation of the system. It is readily evident that these types of systems are expensive. In addition, they rely on mechanical devices of uncertain life and dependability.
Another technique involved the use of a polyethylene sheet to form a vertically extending moisture barrier to prevent movement of moisture from under the foundation and the soil outside the foundation. The polyethylene sheet method was discovered to be expensive and difficult to install, and left no way to naturally add water to the soil under the foundation.
Therefore, according to one embodiment of this invention, an improved foundation and method of constructing the same is disclosed wherein a moisture controlling barrier is placed around the periphery of the foundation for inhibiting the removal of moisture from under the foundation and preventing horizontal capillary migration of moisture through the barrier while allowing percolation of moisture through the barrier and into the soil under the foundation.
According to another embodiment of the invention, an improved method for forming a foundation is disclosed wherein moisture is added to the soil over which the foundation is to be formed until the soil reaches its plastic limit and thereafter, the foundation is constructed over the soil. A vertically extending moisture controlling barrier is formed around the perimeter of the foundation.
The present invention will be readily appreciated by those of ordinary skill in the art as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which: